Free flowing granular material, for instance, sugar, is often stored or contained in silos or bins having rigid walls and bottoms made of metal or some other rigid material. A discharge port is generally provided in the bottom of such a bin or silo, which, when opened, permits the material in the container to flow out. From the discharge port, the material may be conveyed away by a conveying means such as a screw conveyor. If the bottom of the silo or bin, extending from the discharge port to the walls, is flat or horizontal, not all of the free flowing granular material will be discharged through the discharge port by gravity. It is a characteristic of free flowing granular material contained in a flat bottomed bin or silo to stop flowing out the discharge port when the material remaining in the bin is at an angle of repose. The material remaining in the bin after discharge by gravity, forms a cone shape inside the silo. The inner face of this cone shape, formed by the free flowing granular material, extends from the discharge port in the bottom of the silo upward at an angle to the wall of the silo or bin. The angle of repose at which this cone shape occurs and discharge by gravity ends depends on the physical characteristics of the material involved.
To ensure the discharge of the entire contents of a bin, bins have been provided with hopper bottoms. These hopper bottoms are cone-shaped, extending upward from the discharge port at an angle towards the bin or silo walls. The angle at which the hopper bottom projects from the discharge port to the bin wall is sufficient to prevent the material in the bin from resting in a cone shape at an angle of repose and to direct the entire contents of the bin towards the discharge port for removal. With a hopper bottom, all of the material placed in the silo or bin is discharged when the discharge port is opened because it cannot form a cone at an angle of repose. The cone shape of the silo bottom, however, concentrates the weight of the free flowing granular material on a smaller area than a flat bottomed silo or bin of the same size. In addition, a bin with a hopper bottom has a higher center of gravity than the same sized bin with a flat bottom. This higher center of gravity may not be a significant problem in stationary bins, but may be of greater importance in a mobile bin or container. Moreover, a hopper shaped bin is expensive and wasteful of space.
Some prior art has used pneumatically movable flexible membranes inside containers to move the materials in the containers. However none of such prior art has, to applicants knowledge, been even suggestive of using a flexible membrane as the storage bin and pneumatic pressure inside a bag shaped membrane to move stored materials from this angle of repose toward a discharge opening.